A platen glass (original reading surface) of an image reading apparatus of a copying machine and the like is conventionally provided with an openable and closeable platen cover that covers an original placed on the platen glass.
Typically, a thin original is read with the platen cover closed to cut off outside light whereas a thick original such as a book is read with a platen cover open in a so-called sky-shot. In the sky-shot, outside light cannot be cut off by the platen cover and the effect of outside light incident on the platen glass cause a problem that a wrong original range is read and the size of the original is not correctly recognized.
In order to address the problem, Patent Document 1, for example, proposes an image reading apparatus which includes control unit for, each time a scan line is to be read, turning off a light source and causing an image reading section to read outside light information entering a platen with a cover open and with an original placed on the platen, then turning on the light source and causing the image reading section to read an image of the original, and image processing circuit for eliminating the effect of outside light detected from the read image of the original on the basis of the outside light information.
Although the method for detecting the size of the original described in Patent Document 1 can detect the size of the original, the detection method requires, every time a scan line is read, turning off the light source and causing the image reading section to read outside light information entering the platen and then turning on the light source and causing the image reading section to read the image of the original. Accordingly, the method requires a long scanning time.
The present invention has been made in light of these circumstances and an object of the present invention is to provide an image reading apparatus and others which accurately detect the size of an original to read the original while eliminating the effect of outside light incident on a platen glass in a sky-shot, and reduce the time for detecting the size of the original and increase the efficiency.    Patent Document 1: Japanese Laid-open Patent Publication No. 2007-28413